There is no IRC mandate to do so, but the IECC does seem to have this requirement if that document is also in force in your area. If you do insulate the plenum, the IRC does require you to use a vapor barrier on the exterior of the insulation or use duct insulation blankets with integral vapor barrier and tape the seam(s). Duct insulation may not pass through a fire wall or fire rated floor/ceiling. The ductwork passes through the fire rated assembly without insulation, and then you insulate the exposed ductwork on both sides. I just thought I'd add that in case that applied to your garage. If your plenum is made out of ductboard, then it's already insulated so to speak.